UPS 101 - Knowledgebase
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Cascade: The output of the UPS A is fed into the bypass input of UPS B as shown in the drawing.   In the
event that UPS B should malfunction, UPS A is always in a hot-standby position.  In the extremely rare event
that both units fail, the load will be transferred to the bypass source to maintain power to critical loads.

Pros:   The transfer is natural to the design of the UPS.  
Provides very high reliability and maximum redundancy as four total power sources are available to the critical loads:
1) AC Main Input Source  2) UPS A Battery Bank  3) UPS B Battery Bank  4) Bypass Source

Cons: Each unit must be correctly sized to handle the entire load.  

Parallel: UPS or Inverters in a Parallel redundant configuration require both units' outputs to be connected
together.  This is typically done by using a paralleling cabinet which consists of two inputs and a single
output, as well as inductors that allow for load sharing during parallel operation.

Pros:  Can be used to double the output of a UPS and can be a cost effective method when paralleling for capacity.  
Ex: Two 50kva UPS can be paralleled for a total output capacity of 100kva.

Cons:  Not a very reliable means of redundancy.  In an event that one unit unexpectedly transfers to a bypass source, the
remaining unit can potentially be back-fed by the bypass source resulting in cleared fusing or internal damage.  This is
because the bypass source is typically a stiffer source of commercial A/C power.

Modular Systems in Parallel: (Also known as an N+1) Modular systems are designed to provide a low cost
solution with the reliability of a redundant system, however this design falls short.  Their inherent design
doesn't contain the same type of de-rated components as an independent or stand alone UPS. The low
reliability of each individual module requires them to be designed in parallel configurations in case of a
single failure. This in turn, reduces their overall MTBF, and while the initial cost of a modular system may be
less, the cost of ownership and risk of premature failure is much higher.
Understanding Cascade VS Parallel
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